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New Scientist Live

Evolving H7N9 bird flu could close poultry markets

By Debora MacKenzie

Where flu strains mingle

(Image: Peter Adams/Getty)

THERE’S good news and bad news. At last report, there have been no new human cases of H7N9 bird flu in China since 8 May. On the other hand, it seems the virus already has many of the mutations that would allow it to spread easily among us while remaining deadly.

Meanwhile, New Scientist can reveal that China is considering closing live poultry markets permanently in an attempt to stop the spread of the virus.

H7N9 flu has caused 131 known cases in humans, 36 of whom have so far died, since it emerged in February this year. Tests show that while the virus is a very rare infection of birds in live poultry markets, it is circulating and evolving. But the infection is so rare that those birds can’t be passing the virus to enough other birds to keep it circulating. This means it must also be carried, less rarely, by another animal – so far unknown.

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Now Yi Guan at the University of Hong Kong in Pokfulam and colleagues have shown that H7N9 spreads through the air between ferrets, the best test animal for human flu, just like the experimental H5N1 bird flu that caused an outcry in 2012.

Five mutations make H5N1 transmit via airborne droplets between ferrets, and H7N9 already has three of them, but it was not known whether the mutations have the same effect on H7N9. It seems they do&colon; healthy ferrets housed in cages with infected ferrets all came down with the virus, as did three out of four ferrets in a nearby cage, showing it spreads in airborne droplets, though not with the 100 per cent efficiency of ordinary flu.

Guan’s team also found that ferrets spread H7N9 before flu symptoms develop. This means that, as with most flu, we cannot control the spread of H7N9 by isolating people as soon as they show symptoms.

The key now, the team says, is to stop H7N9 becoming more common in poultry, as this would “greatly increase” opportunities to evolve more transmissibility.

The Chinese government has closed poultry markets in cities throughout the eastern provinces affected by the virus. This, and the warmer weather that inhibits survival of the virus, is probably behind the slowdown in human cases, says Keiji Fukuda, head of flu at the World Health Organization. But, he warns, “it is unlikely that the virus has simply disappeared”.

Hua Wang of the Jiangsu Provincial Center for Disease Control and Prevention in Nanjing, China, confirmed this week that markets are a crucial piece of the puzzle, reporting that a human case of the virus in Nanjing was virtually identical to a sample from a poultry market (New England Journal of Medicine, doi.org/mmx).

“The Chinese government is thinking about shutting live markets permanently as a pilot project” in cities like Nanjing and Suzhou, Wang says. If human infections stay low, this could be extended. But the country’s poultry industry is already down &dollar;6.5 billion in lost sales due to H7N9, according to an official estimate. So “there is great pressure to open the markets again”.

This article appeared in print under the headline “Evolving bird flu could close markets”